1. Field of Invention
The present invention concerns a power supply device for electric discharge lamps, for example, a power supply device for electric discharge lamps that turns on a high-luminance high-pressure mercury vapor lamp that is used as a light source for projectors.
2. Description of Related Art
Metal halide lamps and high-pressure mercury vapor lamps have been used as a high-luminance light source.
In recent years, high-pressure mercury vapor lamps in which the mercury vapor pressure has been raised to obtain desired luminance have been effective as light sources for projectors used in luminous flux control devices such as liquid crystals. The amount of mercury sealed in lamps has tended to increase.
Incidentally, the problem arises of a lamp extinguishing over the course of several seconds to dozens of seconds even after successful initiation by an igniter when a lamp is turned on as the amount of mercury sealed in lamps increases.
The inventors conducted empirical observations under various conditions and discovered that mercury condenses on the cathode and sticks during the cooling period when a lamp is extinguished.
This phenomenon can be briefly explained by stating that electrons are readily released from liquid mercury, as is well known, with the result being that arc release becomes possible at extremely low operation voltage of 15 volts to 20 volts, for example, when liquid mercury is present on a cathode.
If discharge should commence while liquid mercury sticks to a cathode, arc discharge would appear first, and mercury on a cathode would rapidly evaporate. At that time, as mercury on the cathode first evaporates, at those sections opposite the cathode, the discharge site gradually would shift toward the base of the cathode. Once mercury has completely evaporated from the cathode, including the base of the cathode, the arc discharge at low operation voltage terminates and it shifts to glow discharge.
The impedance between electrodes is low during arc discharge, but since it rises during glow discharge, comparatively high operation voltage must be supplied to maintain glow discharge. However, if the voltage output from a power supply device cannot accommodate the operation voltage that rapidly rises, the lamp extinguishes at the moment of shift to glow discharge.
Accordingly, the temperature at the cathode tip would rise if glow discharge can be maintained without extinguishing, and a supply of thermions would eventually become possible, at which point the discharge would shift to arc discharge and maintenance of a steady lighting state by the discharge lamp would become possible.
This problem of lamp extinguishing did not materialize much in the past because of the low amount of mercury used. Accordingly, any liquid mercury on a cathode during the period of igniter actuation could scatter and powerful glow discharge could be maintained due to the high voltage generated by the igniter after the end of arc discharge at low operation voltage.
Accordingly, maintenance of glow discharge by continuous actuation of the igniter even after liquid mercury on a cathode had scattered has been considered as well, but such a method would not be practical because the electrode would be depleted and the light tube would blacken.
The means of resolving the problem of mercury sticking to electrodes of high pressure mercury vapor lamps was proposed in Japanese Kokai Publication Hei-10-116590.
In this means, the cathode cooling rate after the lamp extinguishes is retarded in the course of gradual lamp cooling by raising the heat capacity of the cathode or in the vicinity thereof. By so doing, mercury condensation commences from the cathode or the inner surface of the light bulb, and the condensation/sticking of mercury to the cathode is prevented. Thus, even if much liquid mercury should remain in the cathode when the lamp is relit, discharge could easily shift to glow discharge so long as liquid mercury does not stick to the cathode.
However, the prevention of condensation/sticking of mercury on the cathode becomes difficult when the amount of mercury inclusion is increased in the method of raising the heat capacity in the vicinity of the cathode proposed in said gazette, and the sticking of mercury on the cathode cannot be prevented at all, especially if the lamp is set vertically so that the cathode is below and the anode is above. Specifically, the termination of arc discharge at low operation voltage becomes impossible in this method while the igniter is operating, and the operation voltage required for glow discharge after mercury has been completely depleted from the cathode increasingly rises because of the inclusion of large amounts of mercury. This raises the probability of the lamp extinguishing.
In this connection, the vertical lamp arrangement is useful in that it permits the location of devitrification, which can happen within a lamp inclusion body, to be limited to harmless sites depending on where the light is output, and installation so that the cathode is below and the anode is above is useful in that it prevents flickering, which is important depending on the conditions of lamp usage.
In light of the various problems, the object of the present invention is to provide a power supply device for electric discharge lamps in which the extinguishing of a high pressure mercury vapor lamp with a comparatively high amount of mercury inclusion can be completely prevented when mercury is completely evaporated from the cathode.
The present invention uses the following means for resolving the above-noted problems.
The first means provides a power supply device for electric discharge lamps that lights high pressure mercury vapor lamps in which a cathode and anode are disposed in a discharge space enclosed by an inclusion body and in which noble gas as well as 0.15 mg or more of mercury per 1 mm3 of said discharge space are sealed, wherein a switchable connection from the connection state of a simulated arc discharge resistor virtually equal to the arc discharge resistance during arc discharge of the high-pressure mercury vapor lamp to the connection state of simulated glow discharge resistors that have virtually {fraction (1/7)} of the glow discharge resistance during glow discharge of the high-pressure mercury vapor lamp is completed at the output terminal of the power supply device for electric discharge lamps in question, the simulated arc discharge resistor is connected to the power supply device for electric discharge lamps in question, and the simulated glow discharge current in the transient state of switch from the state of flow of simulated arc discharge current to the simulated glow discharge resistor continues to be under 30% of the simulated arc discharge current for less than 10 xcexcs and the duration until the current recovers to at least 70% of said simulated arc discharge current is less than 100 xcexcs.
The second means provides the power supply device for electric discharge lamps of the first means that has a function of controlling the lamp current so that the lamp power reaches a predetermined rated power, and a function of controlling the lamp current so that the lamp current does not exceed a predetermined maximum current, wherein the function of controlling the lamp current so that the lamp current does not exceed the maximum current takes priority over the function of controlling the lamp current so that the lamp power reaches the rated power, and a control function is provided so that the duration of control so as to restore the current to at least 70% of the simulated arc discharge current is more than 50 ms upon switching to the simulated glow discharge resistor and so that this duration will tolerate the rated power to be exceeded.
The third means provides a power supply device for electric discharge lamps that lights high pressure mercury vapor lamps in which a cathode and anode are disposed in a discharge space enclosed by an inclusion body and in which noble gas, 0.15 mg or more of mercury per 1 mm3 of the discharge space and 1xc3x9710xe2x88x927 moles of halogen per 1 mm3 of the discharge space are sealed in said discharge space, wherein a switchable connection from the connection state of a simulated arc discharge resistor virtually equal to the arc discharge resistance during arc discharge of said high-pressure mercury vapor lamps to the connection state of simulated glow discharge resistors that are virtually equal to the glow discharge resistance during glow discharge of said high-pressure mercury vapor lamp is completed at the output terminal of the power supply device for electric discharge lamps in question. Vagxe2x80x2 represents the output voltage of the power supply device for electric discharge lamps and Iagxe2x80x2 represents the simulated glow discharge current upon switching to the simulated glow discharge resistor from the state of flow of simulated arc discharge current by connecting the simulated arc discharge resistor to the power supply device for electric discharge lamps in question. The device in which the cathode surface area is represented by Sc (mm2) has the following characteristics.
(1) The simulated glow discharge current in the steady state is Iagxe2x80x2/0.14xc3x97Sc (A)
(2) The output voltage in the steady state is Vagxe2x80x2/180 (V)
(3) The time required for the output voltage Vagxe2x80x2 to reach 90% of the voltage in the steady state is time xcfx84*170 (xcexcs).
The fourth means provides said power supply device for electric discharge lamps of any one of means 1 to 3 that is provided with a variable output direct current power source that inputs direct current voltage and then applies variably-controlled output voltage to the high-pressure mercury vapor lamps via a smoothing capacitor, wherein the capacitance of the smoothing capacitor is increased during transition to arc discharge following the end of glow discharge.